Astronomy fanatica

last SG · 2005-02-27T00:18:37+00:00

u'll find it on page 4 of this thread

poon cho tang SG

Astronomy Picture of the Day

2005 May 5

Mira: The Wonderful Star

Explanation: To seventeenth century astronomers, Omicron Ceti or Mira was known as a wonderful star - a star whose brightness could change dramatically in the course of about 11 months. Modern astronomers now recognize an entire class of long period Mira-type variables as cool, pulsating, red giant stars, 700 or so times the diameter of the Sun. Only 420 light-years away, red giant Mira (Mira A, right) itself co-orbits with a companion star, a small white dwarf (Mira B). Mira B is surround by a disk of material drawn from the pulsating giant and in such a double star system, the white dwarf star's hot accretion disk is expected to produce some x-rays. But this sharp, false-color image from the Chandra Observatory also captures the cool giant star strongly flaring at x-ray energies, clearly separated from the x-ray emission of its companion's accretion disk. Clicking the Chandra x-ray image of Mira will reveal an artist's vision of this still wonderful interacting binary star system.

Credit:
X-ray Image: M. Karovska (Harvard-Smithsonian CfA) et al., CXC / NASA
Illustration: M.Weiss(CXC)

poon cho tang SG

[email protected]

[email protected] ("SETI at home") is a distributed computing project for Internet-connected home computers, hosted by the University of California, Berkeley, in the United States. SETI is an acronym for the Search for Extra-Terrestrial Intelligence.

[email protected] under classic client (version 3.08)

[email protected] version 4.18

The purpose of [email protected] is to analyze data incoming from the Arecibo radio telescope, searching for possible evidence of radio transmissions from extraterrestrial intelligence. With over five million users worldwide, the project is the most successful example of distributed computing to date. Anybody can participate by running a free program that downloads and analyzes radio telescope data.

It performs three main tests:

- searching for Gaussian rises and falls in transmission power, possibly representing the antenna passing over a radio source
- searching for pulses possibly representing a narrowband digital-style transmission
- searching for triplets, three pulses in a row

While the project has not found any conclusive signs of extraterrestrial intelligence, it has identified several candidate spots for further analysis. On September 1, 2004, an interesting signal SHGb02+14a was announced.

Figures

Since its launch on May 17, 1999, the project has logged over two million years of aggregate computing time. On September 26, 2001, [email protected] had performed a total of 10^21 floating point operations. It is acknowledged by the Guinness Book of World Records as the largest computation in history. With about 500,000 in the system, [email protected] has, by conservative estimates, the ability to compute 100 TeraFLOPS.

How does the system work?

Data is recorded on high density tapes at the Arecibo Observatory in Puerto Rico, about one 35 Gbyte tape per day, then mailed to Berkeley. It is then divided into 0.25 Mbyte chunks which get sent from the [email protected] server over the internet to people around the world to analyze. Arecibo does not have a high bandwidth internet connection, so data must go by postal mail to Berkeley at first.

[email protected] searches for strong narrow band signals. The process is somewhat like tuning a radio set to various channels, and looking at the signal strength meter. If the strength meter goes up, that gets the attention. More technically, it involves a lot of digital signal processing, mostly Fourier transforms at various chirp rates and durations.

The project also searches for pulsing and drifting signals, and signals which match the antenna beam pattern as the telescope slews across the sky. The analysis software can search for signals with about one-tenth the strength of those sought in previous surveys, because it makes use of a computationally intensive algorithm called coherent integration that no one else has had the computing power to implement.

Data are merged in a database using [email protected] computers in Berkeley. Interference is rejected, and various pattern-detection algorithms are applied to search for the most interesting signals.

Software

The [email protected] distributed computing software, available for all major operating systems, runs either as a screensaver or continuously while a user works, making use of otherwise wasted processor power for research. [email protected] was the first popular distributed computing application. After six years of fruitless searching, however, some critics now believe that unused computer cycles could be better spent on projects that have more direct benefits to the human race, such as [email protected]

[email protected], in addition to its altruistic use to aid SETI, is quite useful as a stress testing tool for computer workstations. Since it uses error-correction algorithms to verify the results of the computations, [email protected] is often used to check on the reliability of a computer configuration when overclocking.

There are future plans to get data from the Parkes Observatory in Australia to analyse the southern hemisphere. [email protected] is in the process of transferring to a new software platform called Berkeley Open Infrastructure for Network Computing (BOINC) that will allow testing for more types of signals as well as let users to contribute to other distributed computing projects running on the BOINC platform.

There are minor technical requirements in order to participate in the project. Systems running Windows OS need at least 32 MB of RAM, the ability to display 8-bit graphics in 800x600 resolution, 10 MB of disk space, and an Internet connection. For Macintosh, a PowerPC processor and Mac OS 7.5.5 or later is required. [email protected] software is also supported by UNIX and Linux.

The results of the data processing can be sent automatically, next time the users are on the internet, or it can be set to ask permission before logging on to the internet.

Competitive aspect and [email protected] farms

[email protected] users quickly started to compete with one another in an effort to process the maximium number of work units. Teams were formed to combine the efforts of individual users. The Ars Technica 'Team Lamb Chop' (see link) in the US led the statistics for many years but has recently been overtaken by both SETI.Germany and OcUK OverClockers UK.

Some users were able to run the application on PCs they had access to at work (an act known as Borging, after the assimilation-driven Borg of Star Trek: The Next Generation. Others simply collected large quantities of equipment together at home and created "SETI farms" (typically consisting of motherboard, CPU, RAM and PSU only) arranged on shelves as diskless workstations running either LINUX or Windows 98se "headless" (without a video card).

As with any competition, numerous attempts have been made to 'cheat' the system and claim credit for work that has not been performed. To combat cheats, the organisers send out each work unit multiple times and only stop sending out the same work unit when they receive back results from two or more different users that exactly agree.

Issue #73

poon cho tang SG

Astronomy Picture of the Day

2005 May 6

Fred Espenak's Solar Eclipse
Stephan Heinsius' Solar Eclipse

Hybrid Solar Eclipse

Explanation: April's spectacular geocentric celestial event was a rare hybrid eclipse of the Sun - a total or an annular eclipse could be seen depending on the observer's location. For Fred Espenak, aboard a gently swaying ship within the middle of the Moon's shadow track about 2,200 kilometers west of the Galapagos, the eclipse was total, the lunar silhouette exactly covering the bright solar disk for a few brief moments. His camera captured a picture of totality revealing the extensive solar corona and prominences rising above the Sun's edge. But for Stephan Heinsius, near the end of the shadow track at Penonome Airfield, Panama, the Moon's apparent size had shrunk enough to create an annular eclipse, showing a complete annulus of the Sun's bright disk as a dramatic ring of fire. Pictures from the two locations are compared above. How rare is such a hybrid eclipse? Calculations show that during the 21st century just 3.1% (7 out of 224) of solar eclipses are hybrid while hybrids comprise about 5% of all solar eclipses over the period 2,000 BC to AD 4,000.

Credit & Copyright: Fred Espenak and Stephan Heinsius

poon cho tang SG

Wow! signal

The Wow! signal was a strong narrowband radio signal detected by the astrophysicist Jerry R. Ehman on August 15, 1977 by a SETI project at the Big Ear radio telescope of Ohio State University. The signal was apparently of non-terrestrial and non-solar system origin. It lasted for 72 seconds and has not been detected thereafter. All searches so far for the signal were unsuccessful, both by J. R. Ehman and others. The nature of the signal is therefore unknown and only a few possibilities can be ruled out.

The bandwidth of the signal is less than 10 kHz. Two different values for its frequency have been given, 1420.356 MHz (J. D. Kraus) and 1420.456 MHz (J. R. Ehman), but both very close to the frequency of 1420.405 MHz produced by the hyperfine transition neutral hydrogen in the universe. Also two possible equatorial coordinates are given: R.A. = 19h22m22s ± 5s or 19h25m12s ± 5s, and both dec. = -27°03´ ± 20´ (epoch B1950.0).

Amazed by the signal, how the narrowband it was, and the exact match to the intensity variation a localized signal would produce in the antenna used, J. R. Ehman circled on the computer printout the "6EQUJ5" letter code (the intensities detected are coded by space = intensity 0, "1"-"9" = intensities 1-9, "A" = intensity 10, "B" = intensity 11, and so on) of the intensity variation and wrote the comment "Wow!" on its side. This comment became the name of the signal.

It has been speculated whether interstellar scintillation of a weaker continuous signal similar to the effect of atmospheric twinkling could be a possible explanation (although this still would not exclude the possibility for the signal being artificial in its nature). However, even by using the significantly more sensitive Very Large Array, such a signal could not be detected, while the likelihood of a signal below the Very Large Array level could be detected by the Big Ear radio telescope due to interstellar scintillation is extremely low, less than 1E(-40).

Issue #74

poon cho tang SG

Astronomy Picture of the Day

2005 May 7

NGC 3314: When Galaxies Overlap

Explanation: NGC 3314 consists of two large spiral galaxies which just happen to almost exactly line-up. The foreground spiral is viewed nearly face-on, its pinwheel shape defined by young bright star clusters. But against the glow of the background galaxy, dark swirling lanes of interstellar dust are also seen to echo the face-on spiral's structure. The dust lanes are surprisingly pervasive, and this remarkable pair of overlapping galaxies is one of a small number of systems in which absorption of visible light can be used to directly explore the distribution of dust in distant spirals. NGC 3314 is about 140 million light-years away in the multi-headed constellation Hydra. This color composite was constructed from Hubble Space Telescope images made in 1999 and 2000.

Credit: W. Keel & R. White (Univ. Alabama, Tuscaloosa), Hubble Heritage Team (STScI/ AURA), ESA, NASA

poon cho tang SG

Archaeoastronomy

Archaeoastronomy (also spelled Archeoastronomy) is, as the name implies, the combination of astronomical and archaeological studies. Specifically, it covers the intersection between the two. One aspect of this is the use of the historical record prior to the genesis of the modern astronomical discipline in order to study past astronomical events. It also may involve the use of astronomical knowledge and extrapolation to further our understanding of the historical record. The two disciplines interact constructively. Cultural anthropologists have contributed to the field of archaeoastronomy by studying the astronomy of more modern, historical cultures; this study is termed, ethnoastronomy.

An example of the first sort of archaeoastronomy is the study of the extensive records of ancient China for references to "guest stars". "Guest stars," or star-like objects which appeared in the night sky, were of great interest to the observers of ancient China and were often dutifully recorded. These events have been associated with many transitory phenomena, such as comets and, particularly, supernovae.

An example of the second sort of archeoastronomy is the study of solar, lunar, and stellar alignments of historical monuments. Many claims have been made that the megalithic monument, Stonehenge, represents an "ancient observatory," although the extent of its use in that regard is in dispute. Certainly Stonehenge - and many other ancient monuments - is aligned with particular significance to the solstice and equinox points. Similar claims have been made that the Great Pyramids of Egypt are aligned with the stars in the belt of Orion, in reflection of the significance invested in that constellation by the ancient Egyptians.

During the 1960s, Alexander Thom did a thorough research on megalithic monuments in Britain, and published the results is Megalithic sites in Britain (Oxford, 1967). Apart from arguing for his theory of the megalithic yard, he also argues with statistical methods that a large part of the monuments in Britain are oriented in a way so they can be used as calendars. His theory proposes that the monuments mark points on the horizon where the sun, moon and principal stars rises at seasonal extremes like midsummer, midwinter and the equinoxes.

Some Old World sites where archaeoastronomy is being explored

- Stonehenge and other structures apparently aligned to astronomical bodies and/or events
- Newgrange
- Ballochroy
- Kintraw
- Minard, Brainport Bay, raising methodological issues
- Carnac, the Grand Menhir Brisé
- Sarmizegetusa Regia, Romania

Stonehenge

Some New World sites where archaeoastronomy is being explored

- Chichen Itza, the caracol
- Monte Alban, zenith tube
- Xochicalco, zenith tube
- Uxmal, Venus allignment of the "Governor's Palace"
- Teotihuacan, the pecked-cross circles as survey-markers
- Cahokia, Mirror of the cosmos

Some artifacts that throw light on archaeoastronomy

- Ancient calendars based on astronomical observations
- Antikythera mechanism
- Nebra skydisk

Issue #75

poon cho tang SG

Astronomy Picture of the Day

2005 May 8

CMBR Dipole: Speeding Through the Universe

Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the above all-sky map, radiation in the Earth's direction of motion appears blueshifted and hence hotter, while radiation on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Credit: DMR, COBE, NASA, Four-Year Sky Map

poon cho tang SG

Antikythera mechanism

The Antikythera mechanism is an ancient artifact believed to be an early clockwork mechanism. It was discovered in a shipwreck off the Greek island of Antikythera, between Kythera and Crete, and has been dated to about 87 BC.

The Antikythera mechanism

The wreck was discovered in 1900 at a depth of about 40 m (140 feet), and many statues and other works were retrieved from it by sponge divers. On May 17, 1902 archaeologist Spyridon Stais noticed that one of the pieces of rock had a gear wheel embedded in it.

The mechanism is the oldest known surviving geared mechanism, made from bronze in a wooden frame, and has puzzled and intrigued historians of science and technology since its discovery. The most commonly accepted theory of its function is that it was an analog computer designed to track the movements of heavenly objects. Recent working reconstructions of the device support this analysis. The device is all the more impressive for its use of a differential gear, which was previously believed to have been invented in the 16th century AD.

The late Professor Derek J. de Solla Price, a science historian working at Yale University, published an article on the mechanism in Scientific American in June 1959 while the device was still only partially inspected. In 1973 or 1974 he published an analysis based on gamma ray imaging by Greek archaeologists. He claimed that the device had been built by a Greek astronomer, Geminus of Rhodes. His conclusion was not accepted by experts at the time, who believed that the Ancient Greeks had the theoretical knowledge but not the necessary practical skills.

A partial reconstruction was built by Australian computer scientist Allan George Bromley (1947-2002) of the University of Sydney and Sydney clockmaker Frank Percival. This project led Bromley to review Price's X-ray analysis and to make new, more accurate X-ray images that were studied by Bromley's student, Bernard Gardner, in 1993.

Later, a British orrery maker named John Gleave constructed a working replica of the mechanism. According to his reconstruction, the front dial shows the annual progress of the Sun and Moon through the zodiac against the Egyptian calendar. The upper rear dial displays a four-year period and has associated dials showing the Metonic cycle of 235 synodic months, which equals 19 solar years. A synodic month is the period between two new moons. The lower rear dial plots the cycle of a single synodic month, with a secondary dial showing the lunar year of 12 synodic months.

Another reconstruction was made in 2002 by Michael Wright, mechanical engineering curator for the Science Museum in London, working with Allan Bromley. He analyzed the mechanism using linear tomography, which can create images of a narrow focal plane and thus visualized the gears in great detail.

In Wright's reconstruction, the device not only models the motions of the sun and moon, but those of every celestial body known to the Ancient Greeks: Mercury, Venus, Mars, Jupiter, and Saturn.

This new reconstruction gives credence to ancient mentions of such devices. Cicero, writing in the first century BC, mentions an instrument "recently constructed by our friend Poseidonius, which at each revolution reproduces the same motions of the sun, the moon and the five planets." Such devices are mentioned elsewhere as well.

It also adds support to the idea that there was an ancient Greek tradition of complex mechanical technology which was transmitted via the Arab world, where similar devices are found later, and which formed the foundation of European clockmaking techniques.

Some scientists believe that not only was the device used to track celestial bodies, but to calculate their positions for events or births.

The original mechanism is kept in the National Archaeological Museum in Athens. There is a replica on display at the American Computer Museum in Bozeman, MT.

The Antikythera mechanism, not described in any surviving source, shows that our knowledge of ancient technology is very incomplete. In 1996 the Italian physicist Lucio Russo (professor at Università di Roma "Tor Vergata") published an essay putting new light on the issue. The book has been translated and published in English in 2004 under the title "The Forgotten Revolution : How Science Was Born in 300 BC and Why it Had to Be Reborn".

Issue #76

poon cho tang SG

Astronomy Picture of the Day

2005 May 9

Stars, Dust and Nebula in NGC 6559

Explanation: When stars form, pandemonium reigns. A textbook case is the star forming region NGC 6559. Visible above are red glowing emission nebulas of hydrogen, blue reflection nebulas of dust, dark absorption nebulas of dust, and the stars that formed from them. The first massive stars formed from the dense gas will emit energetic light and winds that erode, fragment, and sculpt their birthplace. And then they explode. The resulting morass can be as beautiful as it is complex. After tens of millions of years, the dust boils away, the gas gets swept away, and all that is left is a naked open cluster of stars.

Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

poon cho tang SG

Armillary sphere

An armillary sphere (also known as spherical astrolabe) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth.

Armillary sphere

Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments, being preferred by Ptolemy. Armillary spheres became popular again in the late middle ages; the Danish astronomer Tycho Brahe (1546-1601) constructed several of such instruments.

Chinese Armillary sphere

Renaissance scientists and public figures often had their portraits painted showing them with one hand on an armillary sphere, which represented the height of wisdom and knowledge.

Armillary spheres were among the first complex mechanical devices. Their development led to many improvements in techniques and design of all mechanical devices.

A representation of an armillary sphere is present in the modern Portuguese flag and has been a national symbol since the reign of Manuel I.

Issue #77

poon cho tang SG

Astronomy Picture of the Day

2005 May 10

The First Image of an Extra Solar Planet

Explanation: It's the faint red object, not the bright white one that might be a historic find. The white object is surely a brown dwarf star. Quite possibly, however, the red object is the first direct image of a planet beyond our Solar System. The intriguing possibility was first reported last year, but many astronomers weren't then convinced that the "planet" was not just a background star. Earlier this year, the 2M1207 star system was imaged twice more in an effort to resolve the issue. To the delight of the scientific team, the objects kept the same separation, indicating that they are gravitationally bound. The faint red object 2M1207b is therefore 100 times fainter, intrinsically, than the bright white brown dwarf 2M1207b -- a characteristic well explained by a planet roughly five times the mass of Jupiter. The discovery - still subject to further conformation - is considered a step toward the more ambitious goal of imaging Earth-like planets orbiting distant stars. The above image was taken with the high-resolution adaptive-optic NaCo camera attached to the 8-meter Very Large Telescope Yepun in Chile.

Credit: NaCo, VLT, ESO

poon cho tang SG

Astrolabe

The astrolabe was invented by Hypatia of Alexandria and was the chief navigational instrument until the 16th century (when the sextant was invented). Some historians credit the invention of the astrolabe to Hipparchus (2nd century BC) while others credit the Persian Fazari (Richard Nelson Frye: Golden Age of Persia. p163). The 15th Century metal highly precise Astrolabe was developed by Abraham Zacuto in Lisbon, from its quite different and imprecise wood arab-used precursor.

A 16th century astrolabe

An astrolabe consists of a circle marked in degrees (similar to a protractor) with a rotating arm attached at its center. When the 0° mark on the circle is aligned with the horizon, and a star (or other celestial body) "sighted" at the end of the movable arm, the position (in degrees) of the star can be read ("taken") off the calibrated circle (hence, "astro" = star + "labe" = to take).

On the plate (mater) are engraved coordinate lines which represent a stereographic projection of the celestial sphere (climate), valid for places at a specific geographic latitude. Over this coordinate grid rotates the rete, which is a framework with spikes whose points represent fixed stars. After adjusting the instrument for the current time, the position of a star can be read from the coordinate grid. Conversely, the instrument can be adjusted to fit the measured position, and the time can be read off the scale.

The astrolabe therefore is a predecessor of the modern planisphere.

An 18th Century Persian astrolabe, maker unknown, from the Whipple Museum of the History of Science in Cambridge. The points of the curved spikes on the front rete plate, mark the positions of the brightest stars. The name of each star being labeled at the base of each spike. The back plate, or mater is engraved with projected coordinate lines.

Stereographical projection was first described by Ptolemy (2nd century). The astrolabe reached the Islamic world in the 8th or 9th century, and it was re-introduced to Europe via Islamic Spain in the 11th century (early Christian recipients of Arab astronomy included Gerbert of Aurillac and Hermannus Contractus). The mathematical background was established by Al-Battani in his treatise Kitab az-Zij (ca. 920), which was translated into Latin by Plato Tiburtinus (De Motu Stellarum).

The English author Geoffrey Chaucer (ca. 1343 - 1400) compiled a treatise on the astrolabe for his son, mainly based on Messahalla. The same source was translated by the French astronomer and astrologer Pelerin de Prusse and others. The first printed book on the astrolabe was Composition and Use of Astrolabe by Cristannus de Prachaticz, also using Messahalla, but relatively original.

In the 15th century, the French instrument-maker Jean Fusoris (ca. 1365 - 1436) started selling astrolabes in his shop in Paris, along with portable sundials and other popular scientific gadgets of the day.

Issue #78

poon cho tang SG

Astronomy Picture of the Day

2005 May 11

The Sombrero Galaxy in Infrared

Explanation: This floating ring is the size of a galaxy. In fact, it is part of the photogenic Sombrero Galaxy, one of the largest galaxies in the nearby Virgo Cluster of Galaxies. The dark band of dust that obscures the mid-section of the Sombrero Galaxy in optical light actually glows brightly in infrared light. The above image shows the infrared glow, recently recorded by the orbiting Spitzer Space Telescope, superposed in false-color on an existing image taken by NASA's Hubble Space Telescope in optical light. The Sombrero Galaxy, also known as M104, spans about 50,000 light years across and lies 28 million light years away. M104 can be seen with a small telescope in the direction of the constellation of Virgo.

Credit: R. Kennicutt (Steward Obs.) et al., SSC, JPL, Caltech, NASA

poon cho tang SG

Astronomical clock

An astronomical clock is a clock with special mechanisms and dials to display the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.

Prague Town Hall astronomical clock

One of the most famous of this type of clock is the Town Hall clock in Prague, Czech Republic. It is also known as the Prague Orloj.

Prague Town Hall astronomical clock. The clock, built in 1410, shows the phases of the moon and the signs of the zodiac. Later additions include a procession of the apostles above and an 1870 calendar below.

The central portion was completed in 1410. The four figures are set in motion at the hour, with Death (represented by a skeleton) striking the time.

On the hour there is a presentation of statues of the Apostles at the doorways above the clock, with all twelve presented at noon.

In 1870 a calendar display was added below the clock.

Astronomical clock face and animated figures of the Prague Town Hall clock

During World War II the clock was nearly destroyed by Nazi fire. The townspeople are credited with heroic efforts in saving most of the parts. It was gradually renovated till 1948. In 1979 the clock was once more cleaned and renovated. According to local legend the city will suffer if the clock is neglected and its good operation is placed in jeopardy.

Other astronomical clocks

Olomouc, the former capital of Moravia in the eastern part of the Czech Republic, also has an impressive exterior astronomical clock on the main town square.

Astronomical clocks of lesser size are found in other countries around the world:

The Science Museum (London) has a scale model of the 'Cosmic Engine', which Su Sung constructed in 1092. This great astronomical clock was about ten meters high (about 30 feet) and was indirectly powered by falling water and Mercury.

The Copenhagen city hall has what is billed as a complete astronomical clock, set in an interior cabinet. The clock was designed over a period of 50 years by amateur astronomer and professional clockmaker Jens Olsen. It was assembled from 1948 to 1955. From 1995 to 1997 the clock underwent complete restoration.

The Palace of Versailles near Paris has a sumptuous rococo table top astronomical clock which took 12 years for a clockmaker and an engineer to build. It was presented to Louis XV in 1754.

The Gros Horloge in Rouen is a famous astronomical clock (14th century), located in the Gros Horloge street.

The Zytglogge in Bern is a famous astronomical clock from the 15th century situated in the Capital of Switzerland.

Issue #79

poon cho tang SG

Astronomy Picture of the Day

2005 May 12

Stars, Galaxies, and Comet Tempel 1

Explanation: Faint comet Tempel 1 sports a fuzzy blue-tinted tail, just right of center in this lovely field of stars. Recorded on May 3rd slowly sweeping through the constellation Virgo, periodic comet Tempel 1 orbits the Sun once every 5.5 years. Also caught in the skyview are two galaxies at the upper left - NGC 4762 and NGC 4754 - both members of the large Virgo Cluster of galaxies. Classified as a lenticular galaxy, NGC 4762 presents an edge-on disk as a narrow gash of light while NGC 4754 is a football-shaped elliptical galaxy. Similar in apparent size, the galaxies and comet make for an intriguing visual comparison, but Tempel 1 is only about 3 light-minutes from planet Earth. The two Virgo cluster galaxies are 50 million light-years away. NASA's Deep Impact spacecraft is scheduled to encounter Tempel 1 on July 4th, launching a probe to impact the comet's nucleus.

Credit & Copyright: Johannes Schedler (Panther Observatory)

poon cho tang SG

Orrery

An orrery is a mechanical device that illustrates the relative positions and motions of the planets and moons in the solar system in heliocentric model. They are typically driven by a large clockwork mechanism with a globe representing the Sun at the center, and with a planet at the end of each of the arms.

A normal mechanical clock could be used to produce an extremely simple orrery with the Sun in the center, Earth on the minute hand and Jupiter on the hour hand; Earth would make 12 revolutions around the Sun for every 1 revolution of Jupiter. Note however that Jupiter's actual year is 11.86 Earth years long, so this particular example would lose accuracy rapidly. A real orrery would be more accurate and include more planets, and would perhaps make the planets rotate as well.

Orrerys are sometimes referred to as planetariums, although generally, planetariums are hemispherical theaters in which images of the night sky are projected onto an overhead surface. Orreries can range widely in size from hand-held to room-sized.

The first orrery was built in 1713 by George Graham. He was supported (as a patron) by Charles Boyle, 4th Earl of Orrery, and thus the name.

Orrerys are usually not built to scale. Some fixed Solar system scale models have been built and are often many kilometers in size.

Issue #80

poon cho tang SG

Astronomy Picture of the Day

2005 May 13

When the Moon Was Young

Explanation: Remember when the Moon was young? It was just last Monday. On May 9th, this slender crescent Moon was recorded at a tender age of 34 hours and 18 minutes. Well, OK ... when calculating the lunar age during a lunation or complete cycle of phases - from New Moon to Full Moon and back to New Moon again - the Moon never gets more than 29.5 days old. Still, a young Moon can be a rewarding sight, even for casual skygazers, though the slim crescent is relatively faint and only easy to see low in the west as the sky grows dark after sunset. Sighting this young Moon last Monday, lucky astronomer Stefan Seip was also treated to a very dramatic telescopic view of an airliner flying in front of the distant sunlit crescent. At a high altitude, the jet's stunning contrails reflect the strongly reddened light of the Sun setting below the western horizon.

Credit & Copyright: Stefan Seip

poon cho tang SG

Planetarium

A planetarium is a theater built for presenting shows about astronomy and the night sky. The plural of planetarium is planetariums or planetaria.

Planetarium of Planetário Calouste Gulbenkian, in Lisbon, part of the Museum of Navy

Overview

Planetariums typically use a large dome shape for the projection screen, with inclined chairs for comfortable viewing "straight up". A large projector in the center of the dome creates the scene, using a number of movable projectors projecting the images of stars or planets onto the screen. The various projectors are geared to provide an accurate relative motion of the sky, and the entire system can be set to display the sky at any point in time.

Planetarium dome

History

Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun, the Moon and the planets. The discovery of the Antikythera mechanism proved that such devices already existed during antiquity.

The first modern planetarium projectors were designed and built by Carl Zeiss in 1924, and have grown more complex. Smaller projectors include a set of fixed stars, Sun, Moon, and planets, and various nebulae. Larger machines also include comets and a far greater selection of stars. Additional projectors can be added to show twilight around the outside of the screen (complete with city or country scenes) as well as the Milky Way. Still others add coordinate lines and constellations, photographic slides, laser displays, and other images. The OmniMax movie system (now known as IMAX Dome) was originally designed to operate on planetarium screens.

In recent years, planetariums or dome theaters have broadened their offerings to include wide-screen or "wraparound" films, all-sky video, and laser shows that combine music with laser-drawn patterns. The newest generation of planetariums has moved to a fully digital projection system, in which a single large projector with a fish eye lens, or a system of digital video projectors around the edge of the dome, are used to create any scene provided to it from a computer. This gives the operator tremendous flexibility in showing not only the modern night sky as visible from Earth, but any other image they wish (including the night sky as visible from points far distant in space and time).

A portable class of planetariums can be set up for programs at schools, for example, on a temporary basis. Easily transported and quickly erected inflatable structures have been used for this purpose.

The term "planetarium" can also be used to describe the projector itself, or other devices to illustrate the solar system, like a computer simulation or an orrery.

Planetarium aboard cruise liner Queen Mary 2

Notable planetariums

- Adler Planetarium, Chicago, Illinois
- Artis Planetarium, Amsterdam
- Cernan Earth and Space Center, Triton College, River Grove, Illinois
- Davis Planetarium at the Maryland Science Center
- Eise Eisinga Planetarium, Franeker, 1774
- Fels Planetarium at the Franklin Institute (Philadelphia, Pennsylvania)
- Charles Hayden Planetarium at the Museum of Science (Boston, Massachusetts)
- Hayden Planetarium, at the Rose Center for Earth and Space, American Museum of Natural History, New York, NY; James Stewart Polshek, architect, 2000.
- London Planetarium, Marylebone Road, London (part of Madame Tussaud's)
- Montreal Planetarium, Montreal, Quebec, Canada
- H.R. MacMillan Space Centre, Vancouver, Canada

McDonnell Planetarium In St. Louis, Missouri. The planetarium was built in 1963 and operated by the city of St. Louis. In 1983 it became part of the St. Louis Science Center, to which it is joined by an enclosed pedestrian bridge across Interstate 64, reopening in 1985 after extensive remodelling. The planetarium features a Zeiss Planetarium Model IX projector capable of showing the locations of 9,000 stars and the planets on an 80-foot (24 m) dome. In addition to the planetarium theater, the building is home to many displays on space sciences on two levels. The shape of the building's unique roof is known mathematically as a hyperboloid of one sheet.

Planetarium computer software

- 3DPlanetarium (Linux) (Replaced by Celestia)
- Celestia (Linux, Windows, Mac OS X)
- KStars (Linux)
- OpenUniverse (Linux, Windows)
- Starry Night (Windows, Mac OS X)
- Stellarium (Linux, Windows, Mac OS X)

Issue #81

poon cho tang SG

Astronomy Picture of the Day

2005 May 14

NGC 3370: A Sharper View

Explanation: Similar in size and grand design to our own Milky Way, spiral galaxy NGC 3370 lies about 100 million light-years away toward the constellation Leo. Recorded here in exquisite detail by the Hubble Space Telescope's Advanced Camera for Surveys, the big, beautiful face-on spiral does steal the show, but the sharp image also reveals an impressive array of background galaxies in the field, strewn across the more distant Universe. Looking within NGC 3370, the image data has proved sharp enough to study individual pulsating stars known as Cepheids which can be used to accurately determine this galaxy's distance. NGC 3370 was chosen for this study because in 1994 the spiral galaxy was also home to a well studied stellar explosion -- a type Ia supernova. Combining the known distance to this standard candle supernova, based on the Cepheid measurements, with observations of supernovae at even greater distances, can reveal the size and expansion rate of the Universe itself.

Credit: Hubble Heritage Team, A. Riess (STScI) NASA

poon cho tang SG

Prague Orloj

The Prague Orloj or Prague Astronomical Clock (Czech: Praský orloj, pron. [prash-skee or-loi]) is a medieval astronomical clock located in Prague, the capital of the Czech Republic. The Orloj is mounted on the southern wall of Old Town City Hall in the Old Town Square and is a popular tourist attraction.

Prague Orloj, the astronomical clock in the Old Town Square of Prague, Czech Republic

The Orloj is composed of three main components: the astronomical dial, representing the position of the Sun and Moon in the sky and displaying various astronomical details; "The Walk of the Apostles", a clockwork hourly show of figures of the Apostles and other moving sculptures; and a calendar dial with medallions representing the months.

History

The oldest part of the Orloj, the mechanical clock and astronomical dial, dates back to 1410 when it was made by clockmaker Mikulas of Kadan and Jan Sindel, the latter a professor of mathematics and astronomy at Charles University. The Prague Orloj was the third clock of its kind, the first having been constructed in Padua in 1344.

Later, presumably around 1490, the calendar dial was added and clock facade decorated with gothic sculptures.

In 1552 it was repaired by Jan Taborsky, clockmaster of Orloj, who also wrote a report on the clock where he mentioned Hanus as maker of the clock. This was a mistake, and was corrected during the 20th century.

The Orloj stopped working many times in the centuries after 1552, and was repaired many times. In the 17th century moving statues were added, and figures of the Apostles were added after major repair in 1865-1866.

The Orloj suffered heavy damage in last days of WW II when Germans directed artillery fire at the Old-Town Hall. The hall completely burned along with wooden sculptures of the Orloj and the calendar dial face of great artistic value made by Josef Manes. The authentic machinery was repaired and the Orloj started working again in 1948, but only after significant effort.

There exists a good deal of misinformation relating to the construction of the Orloj. For a long time it was believed that the Orloj was constructed in 1490 by clockmaster Jan Ruze (also called Hanus) and his assistant Jakub Cech. Another fictitious story involves the clockmaker Hanus being blinded on the order of the Prague Councillors to prevent him from constructing another similar clock.

Astronomical dial

The astronomical dial is a form of mechanical astrolabe, a device used in medieval astronomy. Alternatively, one may consider the Orloj to be a primitive planetarium, displaying the current state of universe.

The astronomical dial

The astronomical dial has a background that represents the standing Earth and sky, and surrounding it operate four main moving components: the zodiacal ring, an outer rotating ring, an icon representing the Sun, and an icon representing the Moon.

Stationary background

The background represents the Earth and the local view of the sky. The blue circle directly in the center represents the Earth, and the upper blue is the portion of the sky which is above the horizon. The red and black areas indicate portions of the sky below the horizon. During the daytime, the sun sits over the blue part of the background and at night it sits over the black. During dawn or dusk, the mechanical sun is positioned over the red part of the background.

Written on the eastern (left) part of the horizon is aurora (dawn in Latin) and ortus (rising). On the western (right) part is occasus (sunset), and crepusculum (twilight).

Golden Roman numbers at the outer ege of blue circle are the timescale of a normal 24 hour day and indicate time in local Prague time, or Central European Time. Curved golden lines dividing the blue part of dial into 12 parts are marks for unequal hours. These hours are defined as 1/12 of the time between sunrise and sunset, and vary as the days grow longer or shorter during the year.

Zodiacal ring

Inside the large black outer circle lies another movable circle marked with the signs of the zodiac and indicating the movement of the stars. The small golden star shows the position of the vernal equinox, and sidereal time can be read on the scale with golden Roman numbers.

Old Czech time scale

At the outer edge of the clock, golden Swbach numerals are set on a black background. These numbers indicate Old Czech Time (or Italian hours), measured starting with 1 at sunset. This ring moves during the year to coincide with the time of sunset.

Sun

The golden Sun moves around the zodiacal circle, thus showing its position on the ecliptic. The sun is attached to an arm with a golden hand, and together they show the time in three different ways:

1. The position of the golden hand over the Roman numerals on the background indicates the time in local Prague time.
2. The position of the sun over the curved golden lines indicates the time in unequal hours.
3. The position of the golden hand over the outer ring indicates the time in Old Czech Time.

Additionally, the distance of the Sun from the center of the dial shows the time of sunrise and sunset.

Moon

The movement of the Moon on the ecliptic is shown similarly to that of the Sun, although the speed is much faster. The half-silvered sphere of the moon also shows the Lunar phase.

Issue #82

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